endoscopic intramuscular dissection for the local resection of rectal cancer. Recent evidence has shown that deep submucosal invasive rectal cancer is associated with low rates of lymph node metastasis and low rates of high-grade histological risk factors, such as tumor budding, lymphovascular invasion, and poor differentiation. Local excision of these rectal lesions represents a potential curative option that enables organ preservation in these patients. Potential interventions for this indication currently include procedures like endoscopic submucosal dissection, endoscopic full thickness resection, and various transanal surgical approaches. However, the ideal technique has yet to be identified. A new approach, endoscopic intramuscular dissection, is performed within the intramuscular plane between the longitudinal and circular rectal muscle wall and offers a high likelihood of achieving a negative vertical margin and accurate evaluation of invasion depth in situations where deep submucosal invasion is suspected or possible. In this video, we present a step-by-step description of endoscopic intramuscular dissection performed for the successful resection of a rectal lesion with deep submucosal invasion. A 31-year-old female underwent a colonoscopy for several weeks of hematochezia, constipation, and diarrhea. Her biopsy demonstrated a 2-centimeter mass on the posterior wall of the distal rectum, with biopsies obtained consistent with adenocarcinoma. Staging via MRI, shown here, confirmed a T1N0 lesion located 4 centimeters from the anal verge. Following team discussion, on-block endoscopic resection for local control, pathological staging, and evaluation for risk of lymph node metastasis was recommended. With the consent of the patient, endoscopic intramuscular dissection was selected for this procedure. A 20-millimeter polypoid mass was located on the posterior wall of the distal rectum. Endorectal ultrasound was performed and confirmed evidence of a T1B lesion. Endoscopic intramuscular dissection was then performed by introducing the endoscope and observing the lesion. Submucosal marking of the borders of the lesion was performed using an electrosurgical knife with spray coagulation settings. Next, a solution of epinephrine, methylene blue, and saline was injected for submucosal lifting at the anal verge. A transverse mucosal incision was then made at the anal margin just above the dentate line to create a stable submucosal tunnel. The submucosal space was entered, and submucosal dissection was started at the anal incision. Following creation of a stable submucosal tunnel, the intramuscular space was injected with solution and lifted adequately. The inner circular muscle layer was then incised, and the intramuscular space was entered. Following incision of the inner circular muscle, several layers could now be observed – the mucosa, the submucosa, the inner circular muscle layer, and the outer longitudinal muscle layer. Dissection of the intramuscular connective tissue was conducted in the oral direction until the endoscope was well beyond the oral margin of the lesion. Dissection of an intramuscular tunnel is shown here. Prophylactic coagulation with a coagulation grasper was performed. The oral margin of the lesion was then injected and lifted adequately. The oral margin of the lesion was incised through the mucosa, into the submucosa, and then into the intramuscular space. The intramuscular dissection was then continued until the tunnel was re-encountered and the lesion was completely dissected from the underlying deep layers. The lesion was then retrieved with a net. The defect was then carefully inspected. There was no evidence of bleeding, full thickness defects, or longitudinal muscle injury. A gross view of the resected specimen is shown here. Pathology confirmed a PT1B moderately to poorly differentiated adenocarcinoma with negative margins. A low power view of the lesion is shown here. High power views showcasing the negative margins are shown here. Clinical Implications Endoscopic intramuscular dissection is a new and promising endoscopic technique for the local resection of T1 rectal cancers that may not be amenable to conventional endoscopic submucosal dissection. Numerous limitations have been previously described for existing rectal lesion excision procedures. Endoscopic mucosal resection rarely obtains negative deep margins and often results in piecemeal samples, which are associated with higher recurrence rates and decreased pathology sample quality. Endoscopic submucosal dissection is also associated with increased risk of positive deep margins. While full thickness approaches such as endoscopic full thickness excision and surgical transanal techniques may enable en bloc resections, disruption of the total mesorectal excision plane is a major concern. A recent prospective study demonstrated high technical success and resection rates of endoscopic intramuscular dissection in 67 patients, with no major adverse events recorded. By providing en bloc specimens while avoiding distortion of the total mesorectal excision plane, EID offers several benefits over existing resection approaches. Finally, future research will be needed to delineate optimal procedural indications and the role of combinatorial adjuvant therapy to maximize endoscopic intramuscular dissection's curative potential.

endoscopic intramuscular dissection

local resection

rectal cancer

organ preservation

submucosal invasion